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Ростовский Государственный Медицинский Университет

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Медицина общая

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47.2.3 Pedicle Screw

The pedicle screw is the strongest implant because it is inserted into the vertebral body.

47.3 Special Surgical Techniques

47.3.1 Correction ofKyphosis [2–6]

The operative surgery for kyphosis is based on cantilever maneuvers and compression forces. Compression causes shortening of posterior column of the thoracic spine. Distraction is not allowed because of the high neurological risk. The standard construction for the treatment of a exible thoracic kyphosis includes instrumentation reaching from T2 to L1. For the rigid kyphosis, prior to the posterior instrumentation, any kind of osteotomy (pedicle subtraction or Smith–Petersen) or anterior disc and ligaments release should be performed when necessary. Cranial implants are inserted using a claw on each side. The claw can be combined with a pedicle screw in T3 and a transverse process hook in T2 or a pedicle hook in T3 and transverse process hook in T2. In either construction, the superior laminar hook can be substituted by a transverse process hook. The important feature of the claw mechanism is the compression between the two superior vertebrae on either side to avoid a pullout of the cranial implants. The claw can also be used by leaving one vertebra in between (for example T2 and T4). If necessary, more than one claw construct per side can be done on the superior part of instrumentation. Pedicle screws or hooks are inserted as preoperatively planed. The preference at the caudal end is the use of pedicle screws. These can be supplemented with laminar hooks to resist the pullout force. Depending upon the curve rigidity, it may be necessary to insert pedicle screws in all vertebrae. Using the rod benders, 6 mm rods are precontoured to the desired sagittal plane kyphosis. The rods are then inserted into the upper pedicle screws or specialized pedicle hooks. With the rods applied in the correct sagittal plane, the claw constructs are locked. The rods are then reduced to the next caudal implants by applying uniform force on each rod with a rod pusher or rod holder. Compression is then applied to the implant in direction of the proximal claw construct. The nut can then be tightened to maintain the achieved correction before moving on to the next distal implants and repeating the procedure. At the end of the construct, two rod holders are applied to the rods, which are gently pushed down together onto the pedicle screws at T12 and L1. Once the rods are reduced onto these distal implants, the connectors and nuts are screwed on and compression is applied against the proximally adjacent xed implant or a rod holder placed in between. Then the nuts are tightened. Finally, two cross-links are applied close to the ends of the instrumentation.

47.3.2 Correction ofScoliosis [1, 6, 7]

The most common pattern of idiopathic scoliosis is the right thoracic curve, and it is an ideal example to illustrate the principles of correction. The surgical treatment of scoliosis is based on many factors: age of patient, curve exibility, Lenke classication (or other), frontal and sagittal balance, quality of bone, bone graft, patient clinical condition, structure of hospital, and availability of implants, as well as the surgeon’s familiarity with the techniques among others. There are many ways to surgically reduce a scoliosis. It is utmost important to correctly apply the principles of correction. The follow technique using USS by Synthes is one option. The proximal and distal end vertebrae are identied on AP standing and lateral bending, fulcrum bending, or traction and lateral standing X-rays. The classic instrumentation of the right thoracic curve extends from T4 to L1. After posterior approach and intraoperative imaging to conrm the appropriate levels, the implants can be inserted, starting from the concave side and going to the convex side. The amount of implants (pedicle screws or hooks) depends upon the preoperative plan. The more implants are used and the more vertebrae are instrumented, if the potential of correction is greater. The foundation of the construct is established with pedicle screws placed in T12 and L1 caudally and in T4 and T5 cranially, on the concave side. On the convex side cranially, the implants are inserted as a claw construct, and it is applied between T4 and T5. Caudally, pedicle screws are inserted also at T12 and L1. The apical vertebra is usually T8 or T9 and is instrumented with pedicle screws, if possible on both sides. Additional instrumentation can usually be placed at alternating levels; however, additional implants must be used in larger and stiffer curves. A 6mm rod template is placed in the desired sagittal plane on the concave side between the T4 and L1 screws. This template is used for the calculation of the nal rod length while keeping in mind that the spine will automatically and passively elongate during the correction maneuvers. The rod template is then removed, and the appropriate size rod is contoured to the desired sagittal plane and cut to length. The rod can then be inserted into the T4 and L1 implants on the concave side, and collars and nuts are applied on both levels, but the nut is only tightened at L1 while maintaining the sagittal orientation of the rod. Using the complex reduction forceps (persuader), the intervening implants are brought to the rod using translation force. It is important not to apply force beyond that which the bone can withstand. In exible curves, the apical implants are brought to the rod with the persuader. If the curve is stiff, do not primarily proceed to the reduction of apical implants and direct your attention to the convex side. The convex-contoured rod is now applied as a kind of lateral cantilever maneuver. The rod is inserted into the proximal claw inserted at T4 and T5. The claw is then compressed, and the nuts are tightened (T4 and T5) to ensure stability of the implants while the rod is main-

47 Overview ofSurgical Techniques andImplants

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tained in a strict sagittal position. The convex rod is then pushed toward the midline using the rod holder to laterally engage the pedicle screws at T6, repeating the procedure at T8, T10, T12, and nally at L1. It is important not to tighten these nuts at this stage because this would prevent further correction on the concave side. At this moment, only the pedicle screw at L1 on concave side and the screws of the claw on the convex side have the nuts tightened to the rod, all the other implants connected to the rods have the nuts loose. The apical screws on the concave side will now have to be translated toward the concave rod using persuader forceps. After completing the coupling of implants to the rod on both sides of the spine in this way, the spine will have passively found its own length. The individually instrumented vertebrae are then sequentially derotated as they are secured to the rod starting from each end. This is achieved by placing a derotation force through the sticks attached to each of the implants, using the L handle and 6 mm socket wrench to tighten the nuts. It is important to hold the end vertebrae in their normal, neutral position prior to commencing this process in order to avoid transference of the torque force from instrumented to the uninstrumented spine. Cross-links are applied at the extremities of the instrumentation. Decortication of the posterior elements and osteotomy of the facet joints are important steps before the bone-grafting procedure.

47.3.3 Stabilization ofFractures

The principles of treatment of fractures of the thoracic spine are to correct the decient part of the injured spine using appropriate forces, support, and stabilization methods. The use of any type of fracture classication will be helpful to choose the principles and proper implant system.

A good example for using these principles is the Universal Spine System—fracture module by Synthes. This system can be used in the middle and low thoracic spine. It is not recommended in the upper thoracic spine because the pedicles are too small and the instrumentation may be too prominent at these levels. In this system, the implants act as a tension band, a buttress, and a neutralization system. USS allows a lordosation, distraction, compression, as well as xation, in a neutral position. Another important point is that the fulcrum of corrective forces can be adjusted by applying half rings. The important features of this system are: Schanz screws, clamps with separate xation for rods, and Schanz screws and the half-ring clamps. The use of Schanz screw allows easy reduction of the vertebral body in the sagittal plane. The USS fracture clamps have separate xations for rods and Schanz screws; it allows a range motion of + or−18° in the sagittal plane of the Schanz screws. Also, it is possible to do compression or distraction independently of

the Schanz screw angle. The half-ring clamps can move the fulcrum of the corrective forces away from the posterior wall of the vertebral body. When all four Schanz screws have been inserted, the rods of the fracture module are applied to the Schanz screws using fracture clamps with the rods lying medially to the Schanz screws. The clamps are left loose.

47.3.3.1 Reduction andFixation ofFractures withIntact Posterior Wall

The posterior ends of the Schanz screws are manually approximated until the desired correction of the kyphosis has been attained. The set screws on the clamps must remain loose so that the clamps can slide freely toward each other during the reduction maneuver. The center of rotation then lies at the posterior edge of the vertebral body. By creating the lordosis, the vertebral body will be distracted anteriorly, and the disc space and disc height can be restored by ligamentotaxis. Place the cannulated socket wrenches over the caudal Schanz screws and tilt them cranially to create lordosis in the spine. The posterior nuts are then locked. The same procedure is performed on the cranial Schanz screw in order to reestablish the correct sagittal plane. The appropriate posterior nuts are tightened to x the angle between the Schanz screws and the rods. At this stage, it is necessary to distract the Schanz screws to reestablish the normal height of the injured disc and vertebra. A half-ring clamp is placed and locked in the center of each rod between the clamps. Distract the spreader forceps and check the procedure with the image intensier. When the desired distraction is obtained, tighten the set screws and remove the rings.

47.3.3.2 Reduction andFixation ofFractures withFractured Posterior Wall

In this type of fractures, there is a danger that the posterior wall fragments might displace posterior into the spinal canal during the correction of the kyphosis by compressing the posterior ends of the Schanz screws. It is important to protect the posterior wall against compression. Distraction is used to reconstitute the height of the vertebral body and disc space.

Two half rings are placed on each of the 6mm rods prior to reduction with the Schanz screws. A distance of 5 mm between the half rings and the clamp is allowed for every 10° of attempted kyphosis correction. When approximating the ends of the Schanz screws, the clamps will soon touch the half rings, and the center of rotation is transferred posterior to the level of the rods instead of the posterior wall. The lordosis is checked with a lateral image intensier view. The posterior-opening nuts are tightened to secure the correction, and the set screws on the clamps are xed. This procedure is repeated for the other Schanz screws. The half rings are then removed. Distraction is performed between the Schanz screws to obtain the height of the vertebral body.

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After fracture reduction and stabilization, anterior surgery may be required for biomechanical purposes in case of signicant vertebral body comminution, osteoporosis, or incomplete clearance of the spinal canal with persistent neurological decit.

47.4 Tips andTricks

• The pedicle screw is the most powerful implant because

of its three-column insertion. An appropriate utiliza-

tion of these implants, therefore, requires an under-

standing of their mechanical properties as well as the

properties of alternative devices and models of

constructions.

• If the placement of a pedicle screw is difcult, it is pos-

sible to open the spinal canal and, through direct visual-

ization of pedicle, introduce the pedicle screw. If in doubt,

instrumentation should be avoided, especially when opti-

mal purchase and placement are in doubt.

• The principle of ligamentotaxis for posterior fragments

reduction of fracture is valid only if the posterior longitudi-

nal ligament is intact. When the posterior longitudinal liga-

ment is disrupted, then indirect decompression of the spinal

canal should not be done using this procedure. Images from

a CT scan or an MRI can demonstrate the disruption of the posterior longitudinal ligament with the sign of a reverse cortical sign of the posterior wall fragment.

• The use of half rings can be avoided by using rod holders.

• The use of sticks or Schanz screw can be substituted by any kind of elongated screw to allow for cantilever force or other systems.

References

1. Winter RB, Lonstein JE.Congenital thoracic scoliosis with unilateral unsegmented bar and concave fused ribs. Spine. 2007;32:E841–4.

2. Cho KJ, Bridwell KH, Lenke LG, et al. Comparasion of smithPetersen versus pedicle subtraction osteotomy for the correction of xed sagittal imbalance. Spine. 2005;30:2030–7.

3. Gill JB, Levin A, Burd T, etal. Corrective osteotomies in spine surgery. J Bone Joint Surg Am. 2008;90:2509–20.

4. Heary RF, Bono CM.Pedicle subtraction astronomy in the treatment of chronic, posttraumatic kyphotic deformity. J Neurosurg Spine. 2006;5:1–8.

5. Macagno AE, O’Brien MF.Thoracic and thoracolumbar kyphosis in adults. Spine. 2006;19(Suppl):S161–70.

6. Mohan AL, Das K.History of surgery for the correction of spinal deformity. Neurosurg Focus. 2003;14(1):e1.

7. Aebi M, Arlet V, Webb JK.AOSPINE manual. Principle and techniques, vol. 1. NewYork: Thieme; 2007.

Stabilization oftheThoracic Spine

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withInternal Fixator

RobertMorrison andUweVieweg

48.1 Introduction and Core Messages

The preparation of the pedicle within the thoracic spine requires precise preoperative planning. The anatomical structures marking the entry point, the pedicle orientation, and diameter must be known prior to the operation. Performing an adequate preoperative planning including CT scans is elemental for planning of the instrumentation and selection of the correct screw placement. The anatomy of the pedicle and the vertebra is quite different in different parts of the thoracic spine (e.g., special anatomy of Th1). In cases of very small pedicle diameters, especially within the middle thoracic spine, only a parapedicular screw placement may be possible. In cases of small pedicle diameter or poor intraoperative radiological picture quality (obese patient, etc.), navigational screw placement is recommended. Due to the lower axial load within the upper thoracic spine and the stabilization through the rib cage, an additional anterior stabilization is not as often necessary as in the lumbar spine.

48.2 Indications

• Fractures of the thoracic spine.

• Degenerative disorders.

• Deformities/scoliosis.

• Tumors or infections of the spine.

R. Morrison (*) Spine & Scoliosis Center, Asklepios Klinik Bad Abbach, Germany e-mail: r.morrison@asklepios.com

U. Vieweg Department of Conservative and Surgical Spine Therapy with Interdisciplinary Spinal Deformities Centre and Rummelsberg Sectional Center, Hospital Rummelsberg, Schwarzenbruck, Germany e-mail: uwe.vieweg@sana.de

48.3 Contraindications

• Osteopenia/osteoporosis (relative contraindication).

• Ongoing infection within the instrumented vertebra (relative contraindication).

• Poor medical condition of the patient (possibly absolute contraindication).

• Small pedicles make transpedicular stabilization impossible (relative contraindication).

48.4 Technical Prerequisites

Fluoroscopy, special cushions (e.g., Wilson Frame etc.), radiolucent operating table, possibly additional intraoperative electrophysiological monitoring (SSEP, MEP). Facultative, a spinal navigation system can be used.

48.5 Planning, Preparation, andPositioning

Preoperative measurement includes the pedicle diameter, and especially the transverse diameter. Within the thoracic spine, this should be done with a CT scan. If the transverse diameter is large enough to carry the screws, the preoperative planning can take place. The correct entry point can be found after identifying the necessary landmarks. These include the facet joint with its borders and the transverse process. Intraoperatively, the entry point is located in the lateral half of the oval area of the pedicle in the AP uoroscopy. This also includes measuring the transverse diameter of the pedicles (Table 48.1). The orientation of the pedicle can be gauged in lateral radiographs or even more precise using CT scans. Sagittal reconstructions of the planning-CT for the cranial-caudal angle and coronary scans for the lateral deviation are advised (Fig.48.1). These two values show a great variation within the thoracic spine [1, 2]. The pedicle length also shows great variations. The correct screw length cannot be gauged intraoperatively in lateral uoroscopy, as the ante-

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Table 48.1 Showing the average pedicle diameter, transverse angle

corresponding to the midline, and the inclination angle of the pedicle orientation corresponding to the superior end plate

Transversal pedicle

Vertebra Gender Th 1 M 8.8 39 23

F 10.4 29 20

Th 2 M 6 35 23

F 6.7 28 20

Th 3 M 4.1 22 22

F 5.3 22 19

Th 4 M 3.9 29 23

F 3.8 19 17

Th 5 M 4.6 24 25

F 4 17 19

Th 6 M 3.6 26 27

F 4 15 24

Th 7 M 4.5 25 24

F 4.6 11 19

Th 8 M 5 29 20

F 4.6 9 18

Th 9 M 5.3 21 18

F 5.5 12 18

Th 10 M 5.6 20 18

F 6 17 17

Th 11 M 8.3 22 20

F 8.8 15 19

Th 12 M 8 15 20

F 9.4 11 18

Adapted from Ebraheim etal. [1]

diameter (mm)

Transversal angle of the pedicle (°)

Inclination angle of the pedicle (°)

rior cortex has a convex shape. When deciding for a screw length, one must keep in mind that 60% of the pullout force is achieved in the pedicle and 15–20% additionally in the cancellous bone of the vertebral body [3]. So the screw should rather be chosen too short than too long.

48.5.1 Anatomical Specications ofthe Thoracic Spine

• Small pedicle diameter (smallest in the midthoracic spine

T3–T8) [4]. The screw diameter should be 75–80% of the transverse pedicle diameter.

• The medial cortex of the pedicles is much stronger than

the lateral wall, making the lateral perforation much more common [5].

• Very small pedicle diameters make transpedicular screw

placement impossible in such cases (Fig.48.1c).

• Plain radiographs in two planes are often not enough to

plan the instrumentation in the thoracic spine; in such cases, a CT scan is necessary.

48.6 Surgical Technique

48.6.1 Approach

• Open access via a midline incision. The incision should be two segments longer than the intended length of fusion. The subcutis is dissected to the fascia, and wound retractors are applied. The fascia is detached on both sides using a diathermy knife close to the bone. Great caution has to be taken, to leave the interspinous ligaments within the cranial segments intact to prevent a PJK (proximal junction kyphosis). The paraspinal structures are retracted with a raspatory. The muscles are retracted to expose the costal processes (Fig.48.2).

• Alternatively, an additional laminotomy can be performed. This is used when a safe identication of the pedicles cannot be found during the operation (rare indication). In such cases, the lamina is resected to display the medial side of the pedicles. Then the screws can be placed as described above.

48.7 Instrumentation

48.7.1 Trajectory

• Entry point can be found at the intersection of the vertical line along the middle of the superior articular process and the horizontal line through the top of the transverse process. Distances to other landmarks can be misleading in the thoracic spine due to the great anatomical variability [1] (Fig.48.2).

• The pedicle is opened with a pedicle awl, followed by consecutive deepening with a pedicle trocar. The length of the screw can be seen on the side of the trocar. To verify the intact pedicle walls, the walls of the canal are tested with a ball-tip pedicle probe.

• Within the thoracic spine, the screws will have a decreasing convergence toward the midline (20–25° in T1 to 5° in T4–12) (Table48.1) (Fig.48.3).

• To make room for the screwhead, it is advisable to resect part of the medial cortex of the transverse process (Red marking in Fig.48.3).

• The screws should be placed parallel to the superior end plate if possible. Alternatively, a slanted introduction is also possible. Thereby, the tip of the screw is aimed toward the anterior edge of the inferior end plate (Choose higher entry point!) (Fig.48.4).

• Parapedicular screw placement [6].

48 Stabilization oftheThoracic Spine withInternal Fixator

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Fig. 48.1 (a) Plain radiograph of the thoracic spine in AP view and (b) corresponding CT scan in axial view showing the pedicle width and ori-

entation (c) Example of very thin pedicles

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Fig. 48.2 Entry point of the transpedicular screw in the thoracic spine,

when planning a screw placement parallel to the superior end plate

R. Morrison and U. Vieweg

Fig. 48.4 Entry point of the transpedicular screw in the thoracic spine

in a lateral view. Angulation depending upon the desired positioning

Fig. 48.3 Entry point of the transpedicular screw in the thoracic spine,

with an angulation of 7–10° toward the midline

In cases of narrow pedicles, a transpedicular trajectory would cause a burst fracture of the pedicles. In such cases, a more lateral entry point is chosen. The trajectory starts at the tip of the transverse process and enters the vertebral body via the costotransversal joints (Figs. 48.5). This technique involves a greater risk of penetration of the pleural cavity.

Fig. 48.5 Superior view of typical extrapedicular screw placement

48.8 Tips andTricks

• Start out by marking the entry points with K-wires or short pins using the uoroscopy in the AP direction.

• The correct positioning and orientation can be veried by adjusting the uoroscopy to where the K-wire is a “point,” which lies clearly within the pedicle.

• The positioning of the patient is especially important when instrumenting the mid thoracic spine (T3–6), as the scapula interferes with the lateral uoroscopy.

• Laying the arms next to the patient’s body can help to lower the scapula. A clear lateral view has to be achieved prior to draping.

48 Stabilization oftheThoracic Spine withInternal Fixator

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References

1. Ebraheim NA, Xu R, Ahmad M, etal. Projection of the thoracic pedicle and its morphometric analysis. Spine. 1997;22:233–8.

2. Vaccaro AR, Rizzolo SJ, Allardyce TJ, etal. Placement of pedicle screw in the thoracic spine. Part one: morphometric analysis of the thoracic vertebrae. J Bone Joint Surg Am. 1995;77:1193–9.

3. Weinstein JN, Rydevik BL, Rauschning WJN.Anatomic and technical considerations of pedicle screw xation. Clin Orthop Relat Res. 1992;284:34–46.

4. Panjabi MM, Takata K, Goel V, et al. Thoracic human vertebrae. Quantitative three-dimensional anatomy Spine. 1991;16: 888–901.

5. Kothe R, O’Holleran JD, Liu W, etal. Internal architecture of the thoracic pedicle: an anatomic study. Spine. 1996;21:264–70.

6. Husted DS, Yue JJ, Fairchild TA, etal. An extrapedicular approach to the placement of screws in the thoracic spine: an anatomic and radiographic assessment. Spine. 2003;28:2324–30.

Transpedicular Stabilization

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withFreehand Technique ontheThoracic Spine

PauloTadeuMaiaCavali

49.1 Introduction and Core Messages

The use of pedicle screws has become popular during the past decade, rst in applications involving the lumbar spine and subsequently in thoracic spine surgery. Pedicle screws also prevent the need to place instrumentation within the spinal canal-like sublaminar wiring or hooks, which create the risk of neurological injury. Transpedicular stabilization (TS) has been shown to resist exion and extension loads, as well as torsional loads better than other devices. Especially in spinal deformity surgery, the use of TS provides better correction and maintenance than system with hooks and wires. Disadvantages of pedicular screws are related to the misplacement of pedicle screws which can lead to disastrous complications such as vascular or neural injuries. Accurate and safe placement of screw within the pedicle is a crucial step during the surgery. There are many proven techniques used to insert pedicle screws, including uoroscopic or radiographic guidance, stereotactic guidance system based

on computed tomography, direct visualization of pedicle with the use of a laminotomy, and the freehand technique (without intraoperative image guidance). The freehand techniques use established surface landmarks and direct palpation of internal pedicle and vertebral structure. The objective of this chapter is to describe the freehand technique for transpedicular stabilization in the thoracic spine.

49.2 Indications

• Deformities such as scoliosis and kyphosis.

• Trauma with fractures and/or dislocations.

• Tumors and other pathologic fractures.

49.3 Contraindications

• Intense osteoporosis.

• Small pedicle with diameter smaller than 4.0mm.

• Inadequate anterior column support.

P. T. M. Cavali (*) Department of Scoliosis os Hospital AACD-Sao Paulo, Sao Paulo, Brazil e-mail: paulo.escolioseaacd@uol.com.br

49.4 Technical Prerequisites

Fluoroscopy, positioning device (e.g., Wiltse frame), intraoperative neuromonitoring with somatosensory-evoked potentials (SSEP), transcranial electric motor-evoked potentials (TMEP), and electromyography (EMG) are some tech-

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Fig. 49.1 Intraoperative monitoring with somatosensory-evoked potentials (SSEP) and transcranial electric motor-evoked potentials (TMEP).

The thoracic nerve roots from T6–T12 are performed with EMG from rectus abdominus muscle

nical prerequisites. The SSEP and TMEP provide evaluation of cord function, and triggered EMG gives information about any contact of screw with neural structures as spinal cord or nerve roots (see Fig. 49.1). Adequate implants and instruments. There are many pedicle screw systems in the market. Most of them are able to stabilize the thoracic spine. In the thoracic spine, the area to set up the instrumentation is smaller than lumbar one; it means that the prole of head of screws, rods, and connectors must t well for each patient to avoid prominence in the skin.

bra in sagittal plane (see Fig.49.2). For deformities surgeries, the level of spine instrumentation and the number and local of screws depend on many features such as classication, stiffness, and magnitude of curve. The patient is positioned prone on a radiolucent operative table. The abdomen and thorax are permitted to hang freely.

49.6 Operative Technique

49.6.1 Approach

49.5 Planning, Preparation, andPositioning

Prior to surgery, the patient’s X-ray is reviewed to assess pedicle diameter, length, and its orientation. Knowledge of normal pedicle morphometry is essential to proper placement of pedicle screw.

The lateral images with the patient in prone position on

the operative table give orientation of screws for each verte-

• The midline posterior approach is the avenue for placement of thoracic instrumentation. It is performed with wide subperiosteal exposure of the posterior bony elements to the level of the transverse processes (see Fig.49.3). This signicantly more wide exposure beyond the facet and out into the transverse process is important to identify all anatomical landmarks.

• Transverse process and the base of superior facet are used as landmarks. With a 2 mm osteotome, approximately

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